1. Field of the Invention
Aspects of one or more embodiments relate to an image forming apparatus capable of improving safety with respect to overheating of a fixing unit thereof that fixes a toner image transferred onto a recording medium, and a controlling method thereof.
2. Description of the Related Art
Generally, an image forming apparatus produces an image on a recording medium in accordance with input signals. A printer, a copier, a facsimile, a multifunction apparatus having combined functions of the above and other similar functions or devices are included in the image forming apparatus.
An electro-photographic image forming apparatus comprises an electrifying unit, a laser scanning unit which is an exposing unit, a developing unit, a transfer unit and a fixing unit, which are all mounted around a photoconductive drum. A surface of the photoconductive drum is electrified to a predetermined potential by the electrifying unit and then exposed to the laser scanning unit, thereby forming an electrostatic latent image thereon. The developing unit develops a toner image according to the electrostatic latent image. The toner image is transferred to a recording medium and fixed on the recording medium by the fixing unit. Next, the recording medium having the image thereon is discharged to the outside.
The fixing unit heats and presses the recording medium upon which the toner image is transferred, thereby temporarily fusing the toner image in the form of powder, and fixing the toner image onto the recording medium.
For this purpose, the fixing unit comprises a fixing roller that fixes the toner on the recording medium, and a pressing roller pressing the recording medium toward the fixing roller. The fixing roller is equipped with a heating lamp mounted in the center thereof and is heated by a radiant heat of the heating lamp.
At one side of the fixing roller, a temperature sensor is installed in contact with an outer circumference of the fixing roller and comprises a thermistor to measure a surface temperature of the fixing roller. The thermistor measures the surface temperature of the fixing roller and transmits the measurement result to a control unit that controls the overall operations of the image forming apparatus. Then, the control unit switches a switching element based on the surface temperature of the fixing roller, thereby controlling power supplied to the heating lamp so that the surface temperature of the fixing roller can be maintained within a predetermined range.
However, in a case when the thermistor malfunctions due to an error or the control unit malfunctions due to factors such as static electricity, the surface temperature of the fixing roller may be measured as lower than the actual temperature or a temperature variation may not be accurately measured. In such cases, the heating lamp would be heated continuously, thereby damaging peripheral parts by overheating the peripheral parts.